Cost is one of the factors that determines the quality of service in the operation of a base station. When the number of subscribers is small in the initial days, a base station uses a basic type of switchable combiner or a redundancy type of switchable combiner.
FIGS. 1A and 1B are schematic diagrams describing conventional basic and redundancy-type switchable combiners, respectively. As shown in the drawings, the conventional basic and redundancy-type switchable combiners include a plurality of power amplifiers 110, input ports 120 and output ports 130. In the basic-type combiner of FIG. 1A, the inputs and outputs of the power amplifier 110 are connected one to one to each other with a radio frequency (RF) cable so that an input should be corresponded to an output.
The redundancy-type switchable combiner, as shown in FIG. 1B, has 4:3 switches in the input and output ports 120 and 130. Therefore, even though one of the three power amplifiers 111, 113 and 115 performs malfunction, the base station can be operated normally without being broken down by switching on a route to a redundancy amplifier 116.
In FIG. 1B, the power amplifiers 111, 113 and 115 perform normal operation and the switches A, B and C are turned on among the 4:3 switches of the input and output ports 120 and 130. The switches A, B and C transmit an RF signal to the out ports 131, 133 and 135.
If one of the power amplifiers 111, 113 and 115 performs malfunction, for example, the power amplifier 113 is assumed to perform malfunction, the switch B corresponding to the power amplifier 113 is turned off at the same time, and it is connected to a redundancy port E. Thus, the reserve power amplifier 116 is operated to maintain the system normal. In this manner, when the combiner is operated in the redundancy type, only three input and output ports are used.
The basic and redundancy types of switchable combiners can be used in the early days when the number of subscribers is small. However, when the number of subscribers becomes large, these types of switchable combiners cannot be used any more due to the low transmission power. To solve this problem, a power dividing/coupling type of switchable combiner is used to increase the transmission power suitably for the increased number of subscribers.
FIG. 1C shows a conventional power dividing/coupling type of switchable combiner. As illustrated in the drawing, the conventional power dividing/coupling type of switchable combiner includes a plurality of power amplifiers 110, a switchable divider 140 and a switchable combiner 150, and the switchable combiner 150 sums up the outputs of two power amplifiers. Therefore, it can produce an output power twice as high as it used to produce with one power amplifier, whereby it can deal with the increased subscribers. However, if a base station uses a basic-type combiner of FIG. 1A or a redundancy-type combiner of FIG. 1B in the initial time and then when the number of subscribers is increased, adopts a power dividing/coupling-type combiner of FIG. 1C to accommodate more channel for the increased subscriber, the cost for changing the combiner becomes a burden on the system operator.
Meanwhile, the switchable combiner 150 is a two-way switchable combiner. When both of the switches A and B are power-on, the switchable combiner 150 is operated in two ways. Otherwise, when only one switch of them is power-on, the switchable combiner 150 is operated in one way.
FIG. 2 is a schematic diagram describing a conventional switchable combiner adopting an average matching method. As shown in the drawing, the conventional switchable combiner is what a switching function is added to a Wilkinson divider. The conventional switchable combiner is operated as described below.
Signals inputted to the input ports Pin1 and Pin2 are outputted through average matching lines 210 to an output port Pout. Here, the inputted signals receive a control signal for controlling the number of power amplifiers to be operated in the base station system, and determine to operate the combiner in one way or two ways based on the number of power amplifiers determined to be operated above.
Accordingly, when two power amplifiers are operated in the base station system, it means that the switches S11 and S21 and the switches S12 and S22 of the switchable combiner are power-on simultaneously and the switchable combiner is operated as a two-way combiner. When one power amplifier connected to the input port Pin1 is operated, it means that the switches S11 and S21 of the switchable combiner are power-on, and the other switches S12 and S22 become power-off. Thus, the switchable combiner is operated as a one-way combiner. Here, impedance Zm of the average matching line 210 is a value between the one-way combination and the two-way combination, and it is obtained from Equation 1 below.
                              Z          m                =                                            Z              0                        2                    ⁢                      (                                          2                            +                              1                                      )                                              Eq        .                                  ⁢        1            
However, the above average matching method raises a serious problem that reflection loss and insertion loss are increased.